Patent application title: Silicon casting apparatus

Abstract:

In a silicon casting apparatus according to the present invention, silicon
melted by electromagnetic induction heating is continuously solidified
using an electrically-conductive bottomless cold crucible and an
induction coil surrounding the cold crucible. The cold crucible is made
of copper alloy containing beryllium (desirably containing beryllium of
0.1 to 5 mass %), whereby the generation of electric-discharge flaw can
be effectively prevented in performing electromagnetic casting. The use
of the silicon casting apparatus according to the present invention can
greatly extend a crucible life to reduce facility costs. Additionally, a
solar-cell silicon ingot can be produced with high quality.

Claims:

1. A silicon casting apparatus in which it includes: (a)an
electrically-conductive bottomless cold crucible in which a portion of
the cold crucible along its axial direction is divided into a plurality
of elements in a circumferential direction; and (b)an induction coil
which surrounds the cold crucible, thereby enabling silicon to be melted
by electromagnetic induction heating using the induction coil and to be
withdrawn downward and solidified,wherein the cold crucible is made of
copper alloy containing beryllium.

3. The silicon casting apparatus according to claim 1, wherein the silicon
casting apparatus is used to produce a solar-cell silicon ingot.

4. The silicon casting apparatus according to claim 2, wherein the silicon
casting apparatus is used to produce a solar-cell silicon ingot.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to a silicon casting apparatus
suitable for production of a solar-cell silicon ingot and the like. The
silicon casting apparatus includes an electrically-conductive
copper-alloy cold crucible divided into a plurality of elements in a
circumferential direction, and is used when silicon as raw material is
melted by electromagnetic induction heating and solidified to perform
continuous casting of a polycrystalline silicon.

[0003]2. Description of the Related Art

[0004]A silicon crystal is used as a substrate material in almost all
currently-produced solar cells. The silicon crystal is classified into a
single crystal and a polycrystal. Usually, solar cells having higher
efficiency in converting incident light energy into electric energy can
be obtained when the single crystal is used as the substrate.

[0005]The single-crystal silicon is produced by the Czochralski method,
and a high-quality dislocation-free crystal can be obtained. However,
when compared with the polycrystalline silicon, solar-cell production
costs are increased since the silicon production costs are increased. On
the other hand, the polycrystalline silicon is usually produced by a
casting method (hereinafter also referred to as "cast method") in which a
molten silicon is solidified by a mold, or a continuous casting method
(hereinafter also referred to as "electromagnetic casting method") in
which electromagnetic induction is utilized. The substrate material can
be produced at low costs in comparison with the single-crystal silicon
substrate produced by the Czochralski method.

[0006]However, since the cast method is an ingot making method of
solidifying the molten silicon with a quartz crucible or a graphite mold,
impurity contamination is generated due to contact of the molten silicon
with the wall of a container such as the crucible. There is also a
problem that a mold lubricant used to prevent fusion between the ingot
and the mold is mixed into the molten silicon. Additionally, since
continuous casting can be hardly performed by the cast method, production
efficiency is inevitably lowered.

[0007]The electromagnetic casting method is a method which has been
developed in order to solve the problems of the cast method. In the
electromagnetic casting method, the silicon crystal can be cast while the
molten silicon scarcely contacts the crucible or the mold.

[0008]For example, as disclosed in Japanese Patent Application Publication
No. 61-52962, a bottomless cold crucible is used in the electromagnetic
casting method. The cold crucible is made of material (usually, copper)
having good electric conductivity and thermal conductivity, being
comprised of a plurality of strip-like elements inside a high-frequency
induction coil, the strip-like elements are electrically insulated from
one other in a circumferential direction, and the inside of the element
is water cooled. A sectional shape of the coil and a sectional shape of a
portion surrounded by the strip-like elements constituting a crucible may
be formed into either a cylindrical body or a rectangular cylindrical
body.

[0009]Silicon as raw material is loaded in the copper cold crucible which
is formed as a melting vessel, and an alternating current is passed
through the high-frequency induction coil. Since the strip-like elements
constituting the cold crucible are electrically insulated from each
other, the current forms a loop in each element, and the current on the
inner wall side of the cold crucible forms a magnetic field in the cold
crucible to heat and thermally melt the silicon in the crucible. An
inwardly force in a direction normal to the surface of the molten silicon
is exerted to the molten silicon in the crucible by interaction of the
magnetic field formed by the current of the cold crucible inner wall and
the skin current of the molten silicon, whereby the silicon is melted
without contacting the crucible.

[0010]When a support pedestal which retains the molten silicon and an
ingot beneath the cold crucible is moved downward while the silicon is
melted in the crucible, the induction magnetic field becomes weaker as
the support pedestal is moved away from a lower end of the high-frequency
induction coil. Therefore, the generated current is lowered to decrease
an amount of heat generation, and solidification progresses upward in a
bottom portion of the molten silicon. In accordance with the downward
movement of the support pedestal, the raw material is continuously loaded
from above the crucible to continue the melting and solidification,
whereby the silicon polycrystal is continuously cast while solidified
unidirectionally.

[0011]In the electromagnetic casting method, the impurity contamination
can be prevented since the molten silicon scarcely contacts the crucible
wall. Since the contamination from the crucible is eliminated,
advantageously, it is not necessary to use a high-purity material as a
crucible material. Additionally, the production costs can be
substantially reduced due to the continuous casting.

[0012]However, in the actual operation, warping or damaging takes place in
slits between the strip-like elements constituting the cold crucible.
Therefore, a frequent repair is required, and a crucible life is short,
which increases facility costs. The slit warping or damaging is caused by
electric discharging which is generated by the contact of the molten
silicon with the electrically-charged crucible surface (that is, surface
of the each strip-like element). The slit warping or damaging is
generally called "electric-discharge flaw". When the slit is degraded due
to the increase in electric-discharge flaw thus rendering the slit
warping or damaging sorely, an eddy current generated in the molten
silicon is weakened to decrease heat quantity for melting the raw
material. The casting becomes unstable to thereby lower the obtained
ingot quality.

[0013]FIGS. 3A to 3C are photographs illustrating progression of
deterioration of the slit portion in the copper cold crucible which is
used for the purpose of comparison in the embodiments described below.
FIG. 3A shows the normal slit portion before the casting is performed,
and FIGS. 3B and 3C show the slit portion after the casting is performed.
FIG. 3B shows the state of the slit portion after being used 6 times in
the casting, and FIG. 3C shows the state of the slit portion after being
used 8 times in the casting. As can be seen from FIGS. 3A to 3C, the
deterioration of the slit portion becomes much distinctive as the
frequency of use thereof in the casting is increased

SUMMARY OF THE INVENTION

[0014]An object of the present invention is to provide a silicon casting
apparatus having a cold crucible which can extend the crucible life while
heat quantity for melting the raw material is ensured by preventing the
generation of the electric-discharge flaw in the surface of each
strip-like element constituting the cold crucible, when the
polycrystalline silicon is continuously cast by solidifying silicon as
raw material after the silicon as raw material is melted by
electromagnetic induction heating using the electrically-conductive
bottomless cold crucible which is divided into a plurality of elements in
a circumferential direction.

[0015]The inventors studied adoption of a crucible made of material having
an excellent resistance to electrically discharging generated by the
contact of the molten silicon with the crucible surface (surface of the
each strip-like element), in other words, material in which the warping
or damaging is hardly generated in the slits, as one of the means for
solving the problems. The reason why they studied adoption of the
crucible is that even if the electric discharging is generated, when the
electric-discharge flaw is hardly generated due to the great resistance
to the electric discharging, the slit warping or damaging is reduced to
extend the cold crucible life, and the increase in facility costs can be
suppressed.

[0016]In addition to the excellent electric conductivity and thermal
conductivity, the conditions which should be possessed by the cold
crucible include excellent hardness, strength, and wear-resistant
property at high temperatures in order to prevent or suppress the
generation of the electric-discharge flaw.

[0017]From such standpoints, the inventors focused attention on a copper
alloy containing beryllium. For example, beryllium copper (a copper alloy
in which beryllium of about 2 mass % and a small amount of cobalt are
contained in copper) defined by JIS H 3270 is utilized as a
high-conductivity spring, a spot welding electrode, and the like, so that
the beryllium copper is considered to be excellent in terms of hardness,
strength, and wear-resistant property at high temperatures when compared
with copper.

[0019]FIG. 1 shows tensile strength and Brinell hardness (either one
indicating a value after a solution heat treatment and aging treatment),
excerpts from Table 1-56 (heat treatment and characteristics of casting
beryllium copper) of page 105 in Non-Patent Document, while a horizontal
axis indicates a beryllium content. In Table 1-56, the beryllium content,
the tensile strength, and the Brinell hardness each are grouped in
certain range. However, for the purpose of convenience, a median in each
numerical range is adopted in FIG. 1. The tensile strength is expressed
while a unit of (kg/mm2) is converted into a unit of (N/mm2).

[0020]As can be seen from FIG. 1, both the tensile strength and the
Brinell hardness are substantially linearly increased with the beryllium
content increasing. Although FIG. 1 shows the test data measured at room
temperature, it is considered that the beryllium copper is excellent in
hardness and strength at high temperatures since the beryllium copper is
used in a welding electrode. Therefore, it is assumed that the tendency
shown in FIG. 1 is sustained at high temperatures.

[0021]Accordingly, the material of the cold crucible is changed from pure
copper to beryllium copper, the cold crucible is prepared, and the
electromagnetic casting is actually performed. As a result, it is
confirmed that the generation of the electric-discharge flaw is prevented
to thereby substantially extend the cold crucible life.

[0022]The present invention is made based on the above-described study
result, and the gist thereof pertains to the silicon casting apparatus
below.

[0023]In accordance with an aspect of the present invention, a silicon
casting apparatus in which it includes: (a) an electrically-conductive
bottomless cold crucible in which a portion of the cold crucible along
its axial length is divided into a plurality of elements in a
circumferential direction; and (b) an induction coil which surrounds the
cold crucible, thereby enabling silicon to be melted by electromagnetic
induction heating using the induction coil and to be withdrawn downward
and solidified, is characterized in that said cold crucible is made of
copper alloy containing beryllium.

[0024]In the silicon casting apparatus according to the present invention,
it is preferable that said copper alloy contains beryllium of 0.1 to 5
mass %, since a cold crucible life extension effect is clearly recognized
without accompanying any difficulty in preparing the cold crucible.

[0025]The silicon casting apparatus according to the present invention, as
a vessel for melting the raw material, includes a bottomless cold
crucible made of copper alloy containing beryllium, in which the crucible
is divided into a plurality of elements in a circumferential direction.
In performing the electromagnetic casting, the silicon casting apparatus
can prevent effectively the generation of the electric-discharge flaw in
comparison with the use of the conventional copper bottomless cold
crucible.

[0026]Therefore, the crucible life can be remarkably extended to
contribute to the facility cost reduction, and the heat quantity for
melting the raw material can be ensured to stably perform the casting.
Additionally, the impurity contamination can be prevented since the
molten silicon scarcely contacts the crucible wall. Therefore, the
silicon casting apparatus according to the present invention is suitable
for the production of the solar-cell silicon ingot requiring high
quality.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a view based on Table 1-56 of Non-Patent Document showing
a relationship between a beryllium content in a Cu--Be alloy and tensile
strength and Brinell hardness;

[0028]FIG. 2 schematically shows a configuration example of a silicon
casting apparatus according to the present invention;

[0029]FIGS. 3A to 3C are photographs illustrating progression of
deterioration of a slit portion in a copper cold crucible which is used
for the purpose of comparison, FIG. 3A shows the normal slit portion
before casting is performed, and FIGS. 3B and 3C show the slit portions
after used 6 and 8 times in the casting, respectively; and

[0030]FIGS. 4A and 4B are photographs illustrating progression of
deterioration of a slit portion in a cold crucible made of a copper alloy
containing beryllium, which is used in the silicon casting apparatus of
the present invention, and FIGS. 4A and 4B show the slit portions after
being used 8 and 16 times in the casting, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031]As described above, a silicon casting apparatus according to the
present invention is characterized in that a cold crucible is made of
copper alloy containing beryllium in an apparatus for producing a silicon
ingot by an electromagnetic casting method using an
electrically-conductive bottomless cold crucible in which a portion
thereof along an axial length is divided into a plurality of elements in
a circumferential direction.

[0032]FIG. 2 schematically shows a configuration example of a silicon
casting apparatus according to the present invention. The silicon casting
apparatus of the present invention is similar to the conventional silicon
casting apparatus in the basic configuration.

[0033]As shown in FIG. 2, a chamber 1 is formed by a water-cooled vessel
having a double walled structure, an upper portion thereof is coupled to
a raw material loading device partitioned by a blocking means 2, and a
withdrawing port 4 for extracting an ingot 3 is provided in a bottom
portion thereof. An inert-gas introducing port 5 is provided at a
sidewall in the upper portion of the chamber 1, and a vacuum suction port
6 is provided at the side wall in a lower portion of the chamber 1.

[0034]A cold crucible 7, an induction coil 8 and an afterheater 9
constituting an electromagnetic casting means are provided in a central
portion of the chamber 1. The cold crucible 7 is a bottomless crucible,
which is formed by a rectangular cylindrical body cooled body made of a
copper alloy containing beryllium and is divided into a plurality of
elements in a circumferential portion while an upper portion of the body
is left without being divided, the elements each being in a strip-like
form. The induction coil 8 is coaxially provided around the outside
circumference of the cold crucible 7 and is connected to a power supply
through a coaxial cable (not shown). Likewise, side by side, the
afterheater 9 is coaxially provided at the lower portion of the cold
crucible 7, and heats the ingot 3 which is withdrawn down from the cold
crucible 7 to give a predetermined temperature gradient in an axial
direction thereof.

[0035]A raw material introducing pipe 10 is provided below a blocking
means 2 disposed at the upper portion of the chamber 1, and a granular
and aggregated silicon raw material 11 loaded into the raw material
introducing pipe 10 is supplied to a molten silicon 12 in the cold
crucible 7. An auxiliary heater 13 made of graphite or the like is
provided in an upward/downward movable fashion, immediately above the
cold crucible 7, such that the auxiliary heater 13 be inserted into the
cold crucible 7 in the lowered state.

[0036]A gas sealing portion 14 is provided below the afterheater 9, and a
withdrawing device 15 is attached to withdraw the ingot 3 downward while
supporting the ingot 3. A diamond cutter 16 as a mechanical cutting means
is provided below the gas sealing portion 14 outside the chamber 1. The
diamond cutter 16 is configured so that the cutter is lowered in
synchronization with a withdrawing speed of the ingot 3 and can
dynamically cut the ingot 3 being withdrawn outside the chamber 1 through
the withdrawing port 4 while following the movement of the ingot 3.

[0037]One of the features of the silicon casting apparatus of the present
invention is that the cold crucible which is a component of the
electromagnetic casting means is made of copper alloy containing
beryllium.

[0038]The reason why the cold crucible is made of copper alloy containing
beryllium is that, as described above, the cold crucible life is markedly
extended while the generation of the electric-discharge flaw (slit
warping or damaging) caused by the contact of the molten silicon with the
crucible surface (surface of the each strip-like element) is prevented to
ensure the heat quantity for melting the silicon as raw material.

[0039]There is no particular limitation to a beryllium content. As shown
in FIG. 1, by analogy with the fact that hardness and strength
substantially linearly increase as the beryllium content increases, when
even a small amount of beryllium is contained, a resistance to the
generation of the electric-discharge flaw should be exhibited to an
extent corresponding to the beryllium content, and the resistance is
increased as the beryllium content is increased. An upper limit of the
beryllium content is restricted from the viewpoints of melting,
production, and processing of the copper alloy.

[0040]However, as far as beryllium of 0.1 mass % or more is contained, the
electric-discharge flaw generation preventing effect is clearly
recognized in the cold crucible. When the beryllium content exceeds 5
mass %, uneven precipitation of the beryllium is likely generated in
welding repair, which causes an undesirable local rupture or cracking of
the beryllium copper. Accordingly, the desirable beryllium content ranges
from 0.1 to 5 mass %.

[0041]As described above, the material to be used for making the cold
crucible included in the silicon casting apparatus of the present
invention is a copper alloy containing beryllium. However, the copper
alloy may contain alloy elements other than beryllium, as long as it has
the excellent electric conductivity and thermal conductivity and the
electric-discharge flaw generation preventing effect is not impaired. For
example, the copper alloy may contain a small amount of cobalt and nickel
in addition to beryllium. When the small amount of cobalt is contained, a
crystal grain growth preventing effect and the like is recognized in the
solution heat treatment.

[0042]For the copper alloy containing beryllium, "beryllium copper" is
defined in JIS H 3270, and "beryllium copper for spring" is defined in
JIS H 3130. A chemical composition of the beryllium copper includes Be:
1.8 to 2.00% ("%" means "mass %", hereinafter), Ni+Co: 0.20% or more,
Ni+Co+Fe: 0.6% or less, and Cu+Be+Ni+Co+Fe: 99.5% or more. A chemical
composition of the beryllium copper for spring includes Be: 1.60 to
1.79%, and other elements are identical to those of the beryllium copper.
In the silicon casting apparatus of the present invention, an equivalent
to this beryllium copper or to that beryllium copper for spring may be
used as material for the cold crucible.

[0043]In order to produce the silicon ingot by the electromagnetic casting
method using the silicon casting apparatus of the present invention shown
in FIG. 2, silicon as raw material 11 is loaded in the cold crucible 7
made of copper alloy containing beryllium, and an alternating current is
passed through the induction coil 8. Therefore, as described above, the
magnetic field is formed in the cold crucible 7 to heat and melt silicon
11 as raw material. At this point, the inward force in a direction normal
to the surface of the molten silicon 12 is applied to the molten silicon
12, and the molten silicon 12 is melted without contacting the cold
crucible 7, thereby preventing the contamination of the ingot 3 incurred
by the contact with the cold crucible 7.

[0044]Then, as described above, while silicon as raw material 11 is melted
in the cold crucible 7, the withdrawing device 15 which retains the
molten silicon 12 and the ingot 3 in the lower portion of the chamber 1
is moved downward to go on with the solidification of the molten silicon
12, and the silicon as raw material 11 is continuously loaded from above
the cold crucible 7 to continue the melting and solidification, which
allows the continuous casting of the silicon polycrystal.

[0045]Thus, the use of the silicon casting apparatus of the present
invention including the cold crucible whose material is changed from the
pure copper to the copper alloy containing beryllium can prevent the
generation of the electric-discharge flaw to markedly extend the cold
crucible life. Additionally, the heat quantity for melting the raw
material is ensured to stably perform the casting, and the impurity
contamination can be prevented since the molten silicon scarcely contacts
the crucible wall. Therefore, the high-quality silicon ingot suitable for
the solar-cell substrate material can be produced.

[0046]Furthermore, the following additional effect can be also obtained
since mechanical strength of the crucible is remarkably increased.

[0047]The copper alloy containing beryllium is an alloy having a large
precipitation hardening property, and the mechanical strength (tensile
strength) is remarkably improved by performing a proper heat treatment.
The pure copper (Cu: not less than 99.9%) has the tensile strength of
about 275 N/mm2, which is designated as an alloy number C1100 in JIS
H 3100. On the other hand, the beryllium copper after an aging hardening
treatment has the tensile strength of about 1400 N/mm2, which is
designated as an alloy number C1720H therein, and the tensile strength of
beryllium copper is usually 4 times that of the pure copper or more.

[0048]Accordingly, in the case where a 345 mm-square ingot is cast using
the pure-copper cold crucible (hereinafter also referred to as "mold"
since the crucible has the rectangular cylindrical body), a 505 mm-square
ingot can be cast without changing a thickness of the mold by using the
mold made of beryllium copper. This can be realized by the remarkable
improvement of the mechanical strength of the mold material, and the cost
saving effect by virtue of material change is prominent in preparing the
expensive mold.

[0049]Since the thickness of the cooled mold can be thinned, a
magnetic-field loss incurred by the thickness of the mold is decreased,
and the transfer of the magnetic field toward the inside of the mold from
the high-frequency induction coil disposed around the outer circumference
of the crucible is increased, which can advantageously enhance the heat
generation efficiency.

EXAMPLES

[0050]The electromagnetic casting was performed using the silicon casting
apparatus of the present invention having the schematic configuration of
FIG. 2, and the state of the slit deterioration associated with the
repetitive use of the cooled mold and the mold life were investigated.
Given that the use of the mold from the start to the end in the
continuous casting is deemed to be one (1) time in the frequency of use,
the mold life was evaluated in terms of the frequency of use in the
casting.

[0051]The crucible made of beryllium copper equivalent defined in JIS H
3270 was used as the cold crucible made of copper alloy containing
beryllium.

[0052]FIGS. 3A to 3C illustrate the progression of the deterioration of
the slit portion in the copper cold crucible which is used for the
purpose of comparison. FIG. 3A shows the normal slit portion before
casting is performed, and FIGS. 3B and 3C show the slit portions after
being used 6 and 8 times in the casting. The deterioration aggravates
considerably in the slit portion shown in FIG. 3C, and it is determined
that the FIG. 3C slit portion should indicate a service-life limit of the
crucible. That is, the 8-time use is regarded as the copper cold crucible
life when evaluated in terms of the frequency of use in the casting.

[0053]FIGS. 4A and 4B illustrate progression of deterioration of the slit
portion in the cold crucible made of copper alloy containing beryllium
(the beryllium-copper crucible), which is used in the silicon casting
apparatus of the present invention. FIG. 4A shows the slit portion after
being used 8 times in the casting and FIG. 4B shows the slit portion
after being used 16 times in the casting.

[0054]The photographs of FIGS. 4A and 3C differ remarkably from each other
in the progression of the deterioration of the slit portion although the
slit portions are used 8 times. In the cold crucible made of copper alloy
containing beryllium, it is found that the generation of the
electric-discharge flaw is effectively prevented. However, as shown in
FIG. 4B, when the frequency of use reaches 16 times, the partial slit
warping is remarkably observed even in the copper alloy containing
beryllium cold crucible. In this case, it is evaluated that the mold life
is 16 times.

[0055]As can be seen from the results shown in FIGS. 3A to 3C, 4A, and 4B,
in performing the electromagnetic casting, the use of the silicon casting
apparatus of the present invention including the bottomless cold crucible
made of the copper alloy containing beryllium can prevent the generation
of the electric-discharge flaw to greatly extend the crucible life. The
same holds true for the cylindrical mold, not only the cold crucible.

[0056]Thus, the silicon casting apparatus according to the present
invention includes the bottomless cold crucible made of the copper alloy
containing beryllium, and the generation of the electric-discharge flaw
can be effectively prevented in performing the electromagnetic casting.
The use of the silicon casting apparatus of the present invention can
greatly extend the crucible life to reduce the facility costs.

[0057]The heat quantity for melting silicon as raw material is ensured to
stably perform the casting, and the impurity contamination from the
crucible wall is prevented, so that the high-quality silicon ingot can be
produced. Accordingly, the silicon casting apparatus of the present
invention can be suitably utilized in the production of the
polycrystalline silicon used for the solar-cell substrate material in
which high quality is demanded.